package LanQiao.T1T3;

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        long n;
        n=sc.nextLong();
        double result=n*n/2.0+n/2.0;

        System.out.printf("%.0f",result);
    }
}
